High speed laser drivers for optical communication deliver electrical signals to a directly coupled semiconductor laser. Such electrical signals consist of a high speed switching signal, plus a direct current (DC) bias current. The bandwidth of drivers depend on the parasitics at the output node and output impedance of the bias circuit.
Currently, in order to prevent the degradation of the signal quality due to impedance mismatching between the semiconductor laser and the laser driver, laser drivers are configured to adjust the output impedance to match the transmission impedance by providing a termination resistor. However, such a fixed resistance provides less than ideal output impedance stability and bandwidth control. The output impedance depends on the output impedance of the bias current source which depends on process and supply voltage, so having a fixed resistor might not be optimum at all operating conditions. Adding the control over this impedance enables optimization of the termination resistance to obtain the best performance.
As such, there is a need in the art for an improved circuit design for high speed laser drivers for optical communication having improved output impedance stability and improved bandwidth control.